The present invention relates to a piezoelectric ceramic composition and a piezoelectric device and, more particularly, to a lead-free piezoelectric ceramic composition having a high heat resistance and a high piezoelectric distortion constant and a piezoelectric device using the piezoelectric ceramic composition.
Most of piezoelectric ceramic materials currently in practical use contain lead as typified by lead titanate (PT) and lead zirconate titanate (PZT). The lead-containing piezoelectric ceramic materials however raise a concern about the environmental effects of their lead components. Further, the lead-containing piezoelectric ceramic materials have a Curie point (Curie temperature) of about 200 to 500° C. and lose their piezoelectric properties at temperatures higher than or equal to the Curie point so that it is difficult to use the lead-containing piezoelectric ceramic materials for piezoelectric ceramic sensors whose operating temperatures are generally 500° C. or higher. There is a need for lead-free piezoelectric ceramic usable at 500° C. or higher with less environmental effects.
As such lead-free piezoelectric ceramic, a bismuth layer-structured ferroelectric material Na0.5Bi4.5Ti4O15 (NBT) is known. (See Patent Publications 1 and 2 and Non-Patent Publications 1 and 2.) The bismuth layer-structured ferroelectric material NBT is expected as lead-free piezoelectric ceramic usable under high-temperature conditions due to the fact that the Curie point of the bismuth layer-structured ferroelectric NBT is about 670° C. and higher than the Curie point of the PT and PZT materials.
[Patent Publication 1] Japanese Laid-Open Patent Publication No. S50-67492
[Patent Publication 2] Japanese Laid-Open Patent Publication No. H11-29356
[Non-Patent Publication 1] “Piezoelectricity in Ceramics of Ferroelectric Bismuth Compound with Layer Structure”, S. Ikegami and I. Ueda, Japanese Journal of Applied Physics, 13 (1974), p. 1572-1577
[Non-Patent Publication 2] “Grain-Oriented and Mn-Doped (NaBi)(1-x)/2CaxBi4Ti4O15 Ceramics for Piezo- and Pyrosensor Materials”, T. Takenaka and K. Sakata, Sensor and Materials, 1 (1988), p. 35-46